Make fixes and improvements

- Document more compiler flags
- Expose new __print_maps() api
- Better overflow checking in mmap()
- Improve the shell example somewhat
- Fix minor runtime bugs regarding stacks
- Make kill() on fork()+execve()'d children work
- Support CLONE_CHILD_CLEARTID for proper joining
- Fix recent possible deadlock regression with --ftrace

libc/log/leaks.c

@@ -108,7 +108,7 @@ noasan void CheckForMemoryLeaks(void) {
     }
     malloc_inspect_all(OnMemory, 0);
     kprintf("\n");
-    PrintMemoryIntervals(2, &_mmi);
+    __print_maps();
     /* PrintSystemMappings(2); */
     /* PrintGarbage(); */
     __restorewintty();